1. Field of the Invention
The present invention relates to ink jet printers, and, more particularly, to a method of printing using an ink jet printer to inhibit the formation of a print artifact.
2. Description of the Related Art
Color ink-jet printers commonly use a tri-color cyan, magenta and yellow ink cartridge, and a separate single color black ink cartridge. Each cartridge contains multiple orifices through which the distribution of ink onto a page can be controlled. Since the majority of printed documents contain primarily black text, the number of black orifices is normally several times larger than the number of orifices for each of the cyan, magenta, or yellow inks. For example, the tri-color cartridge may contain 16 orifices each for cyan, magenta, and yellow, and the black cartridge may contain 48 black orifices. For manufacturing reasons, there typically exists a four-orifice tall gap between the cyan and magenta orifices, and another four-orifice tall gap between the magenta and yellow orifices.
To render a full color image, the tri-color and black cartridges are passed across a print medium such as paper in a horizontal direction perpendicular to the vertical direction of alignment of the orifices in the cartridges. Between passes of the cartridges, the print medium is moved in the advance direction. For a raster, or row of printed dots, which contains at least one of each of cyan, magenta, yellow, and black dots, the color cartridge must be passed at least three times, once passing to deposit any cyan dots present in the given raster, once passing to deposit any magenta dots, and once passing to deposit any yellow dots. Additionally, the black cartridge must be passed at least once. Of course, for any pass of the color head all 48 color orifices can be used, depositing the cyan, magenta, and yellow inks at different raster locations. The most time efficient technique for depositing ink therefore causes the tri-color cartridge to pass across each raster of the page at most three times, and the black cartridge at most one time.
With a conventional method of printing with a tri-color ink jet printer as described above, objectionable print artifacts may occur because of the sequencing and/or timing between the placement of the different color ink dots on the print medium. For example, print artifacts such as horizontal color/black banding or intercolor bleeding may occur.
One known solution to the problem of a color/black banding artifact is known as shingling or interlaced printing. For a 50% shingling mode (i.e., 2-pass or 50% interlace level), approximately 50% of the dots are placed on any given pass of the cartridge. The candidate dots are selected according to a checkerboard pattern. The remaining 50% of the dots are placed on a subsequent pass of the cartridge. For a raster which contains cyan, magenta, yellow, and black dots, the color cartridge must be passed at least six times, twice depositing any cyan dots present in the given raster, twice depositing any magenta dots, and twice depositing any yellow dots. Additionally, the black cartridge must be passed at least twice.
Another type of print artifact which may occur during printing and is not associated with the sequencing and/or timing between placement of the different color ink dots on the print medium is related to the paper transport system which transports the paper through the ink jet printer in the advance direction. The paper transport system must accurately align the paper in the advance direction at a plurality of locations allowing the cartridges to be scanned across the paper at a plurality of corresponding rasters. The paper transport system typically includes a plurality of rollers which are driven by a plurality of corresponding gears. The gears include a limited degree of manufacturing tolerances allowing sufficient clearances and preventing binding therebetween during use. These manufacturing tolerances may cause errors in the exact placement of each raster relative to a registration location of the paper. Moreover, the stepper motor used to drive the gears includes certain inherent errors associated with movement of the motor between steps. These errors are referred to as "step-to-step errors" and are specified by the manufacturer. Since the stepper motor and gears are rotated, these errors tend to repeat on a cyclical basis, dependent upon the particular rotational orientation of the gears as the gears rotate. The errors associated with the paper transport system therefore tend to be harmonic in nature, dependent upon the particular rotational orientation of the gears. Thus, a print artifact having a "rainbow effect" in color samples and gray scale darkness shifts in monochrome samples may occur in the print image on the paper.
What is needed in the art is a method of printing which inhibits the formation of print artifacts associated with harmonic errors in the paper transport system of the ink jet printer.